1. Field of the Invention
The invention relates to a process for treating one or more substrates with a plasma, as well as to an apparatus for carrying out the process. More specifically, the invention relates to such a process and apparatus wherein the substrates are substantially uniformly treated by the plasma.
2. Description of Prior Art
Cold (thermodynamically non-equilibrium) plasmas, excited by a high frequency electromagnetic field in a gas or vapor maintained at a low pressure (partial vacuum) are being used in various industrial processes for treating a variety of substrate surfaces. A major user of cold plasmas is the electronic components industry. In this industry, plasmas are used for a variety of processes involving deposition of thin film materials (referred to as plasma-assisted chemical vapor deposition, PACVD). Plasmas are also used in this industry for the selective removal of material by plasma-generated active chemical species. The latter process is generically known as "plasma etching" or "dry etching".
Many plasma reactor systems for carrying out treatments such as deposition and etching operations can be found in the patent and technical literature. Most of these operate in a regime in which the frequency of the applied electromagnetic field, .omega., lies in the range extending from audio frequencies, typically kHz to tens of kHz, to radio frequencies, i.e., up to about 27.12 MHz.
A particularly favourable frequency range at which to conduct plasma treatments such as deposition and etching operations is the microwave range (.omega.&gt;300 MHz), for example, at the ISM frequency of 2.45 GHz. The reason that this range is favourable, as explained in an article by Michael R. Wertheimer and Michel Moisan, J. Vac. Sci. Technol. November 1985, is that the concentration of plasma-generated active species is much higher in this frequency range than at lower frequencies. This allows one to conduct a given plasma treatment (for example, deposition or etching operation) much more rapidly--a distinct advantage for most industrial processes.
A method and apparatus which can be used for plasma treating a substrate is taught in U.S. Pat. No. 3,814,983, June 4, 1974, Weissfloch et al. In this apparatus, a slow wave structure is used for applying microwave energy to partially ionize a gas in order thereby to provide a plasma. A problem with this apparatus and method is that, as the microwave power density at the input end of the slow wave structure tends to be greater than the microwave power density at the output end thereof, treatment (for example, coating or etching) of the substrate will not be uniform along the length of the slow wave structure. One approach to overcoming this problem is taught in U.S. Pat. No. 4,521,717, June 4, 1985, Kieser. In this approach, two slow wave structures are used each arranged at opposite acute setting angles to the surface of the substrate carrier. It is noted that this approach is also suggested in the '983 patent.
With this approach, it is necessary to use at least two slow wave structures.
The present application teaches a different approach for solving the same problem, which has the advantage of using a single slow wave structure.